This application claims the priority of Japanese Patent Application No. 2000-185281 filed on Jun. 20, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens and optical pickup apparatus for recording and reproducing signals, which are commonly usable for two or more kinds of optical recording media. In particular, the present invention relates to an objective lens which, when light beams for irradiating the optical recording media have wavelengths different from each other according to the kinds of optical recording media, can efficiently converge the respective light beams onto their corresponding optical recording media; and an optical pickup apparatus using the same.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other.
Since disc thickness values are strictly defined in the two optical recording media exemplified above, it is necessary for the individual wavelengths of light for reproducing or recording to be converged at respective predetermined positions different from each other.
Under such circumstances, a system in which two objective lenses having converging actions different from each other are made switchable depending on the optical recording media subjected to reproducing or recording has been known. However, it complicates the structure of optical pickup apparatus and opposes the demands for compactness and lower cost.
On the other hand, it has recently been known to use a multifocal lens composed of a diffraction optical element or a plurality of zonal areas for an objective lens part of an optical pickup apparatus (Japanese Unexamined Patent Publication No. 8-62493, Japanese Unexamined Patent Publication No. 9-145994, and the like).
In the apparatus using the above-mentioned diffraction optical element, however, a light component of an order different from that contributing to the recording and reproducing emitted from the diffraction optical element becomes noise light. In the apparatus using the above-mentioned multifocal lens, on the other hand, the light transmitted through a circular strip division other than that contributing to the recording and reproducing at that time becomes noise light. Due to these noise light components, the S/N ratio at the recording and reproducing deteriorates.
In view of the circumstances mentioned above, it is an object of the present invention to provide a compact, inexpensive objective lens with a favorable S/N ratio for optical recording media, which can favorably converge two wavelengths of light onto their corresponding optical recording media having thickness values different from each other without complicating the structure of optical pickup apparatus; and an optical pickup apparatus using the same.
The present invention provides an objective lens, composed of two lenses disposed in a luminous flux, for optical recording media,
wherein the two lenses are constituted by respective materials different from each other;
wherein the two lenses satisfy the following conditional expressions (1) and (2) respectively with respect to wavelengths xcex1 and xcex2 of light:
N1(xcex1)≅N2(xcex1)xe2x80x83xe2x80x83(1) 
N1(xcex2)xe2x89xa0N2(xcex2)xe2x80x83xe2x80x83(2) 
where
N1(xcex1) is the refractive index of the first lens with respect to wavelength xcex1 of light;
N1(xcex2) is the refractive index of the first lens with respect to wavelength xcex2 of light;
N2(xcex1) is the refractive index of the second lens with respect to wavelength xcex1 of light; and
N2(xcex2) is the refractive index of the second lens with respect to wavelength xcex2 of light; and
wherein each lens surface has a form configured so as to suppress aberration in the objective lens as a whole.
Also, the present invention provides an objective lens, composed of three lenses disposed in a luminous flux, for optical recording media,
wherein at least adjacent lenses in the three lenses are constituted by respective materials different from each other;
wherein the three lenses satisfy the following conditional expressions (3) and (4) respectively with respect to wavelengths xcex1 and xcex2 of light:
N1(xcex1)≅N2(xcex1) and N2(xcex1)≅N3(xcex1)xe2x80x83xe2x80x83(3) 
N1(xcex2)xe2x89xa0N2(xcex2) and N2(xcex2)xe2x89xa0N3(xcex2)xe2x80x83xe2x80x83(4) 
where
N1(xcex1) is the refractive index of the first lens with respect to wavelength xcex1 of light;
N1(xcex2) is the refractive index of the first lens with respect to wavelength xcex2 of light;
N2(xcex1) is the refractive index of the second lens with respect to wavelength xcex1 of light;
N2(xcex2) is the refractive index of the second lens with respect to wavelength xcex2 of light;
N3(xcex1) is the refractive index of the third lens with respect to wavelength xcex1 of light; and
N3(xcex2) is the refractive index of the third lens with respect to wavelength xcex2 of light; and
wherein each lens surface has a form configured so as to suppress aberration in the objective lens as a whole.
Preferably, the two or three lenses are cemented together. The cemented surface of the two or three lenses may be made aspheric.
The wavelength xcex1 may be one of wavelengths of 650 nm and 780 nm for recording and reproducing one of DVD and CD-R, whereas the wavelength xcex2 is the other of wavelengths of 650 nm and 780 nm for recording and reproducing the other of DVD and CD-R.
The present invention provides optical pickup apparatus comprising the above-mentioned objective lenses for optical recording media.